1. Field of the Invention
The present invention relates to a method for preparing polyvinyl chloride sheets suitable for a membrane press forming such as vacuum press forming or air-pressure forming. More particularly, the present invention relates to decorative laminated sheets of polyvinyl chloride for membrane press forming, which are stain-resistant, scratch-resistant and very glossy.
2. Description of the Related Art
Since polyvinyl chloride has superior properties such as being an inexpensive general-purpose resin, having excellent forming properties, and being easy to add color to and print on, it is widely used for membrane press forming such as vacuum press forming or air-pressure forming. In such cases, the following processes are usually conducted: sheets of polyvinyl chloride of the desired thickness, colored or surface printed a desired color, are heated to the vicinity of the softening point; the heated sheets are placed over an adherend in a predetermined form, e.g. wooden matter which will form the door of a kitchen set; the adherend is covered by an additional membrane having unrestricted expansion and contraction, e.g. a rubber membrane, which is pressurized by air or liquid, so that the surface of the adherend is covered uniformly by the polyvinyl chloride sheet.
The surfaces of polyvinyl chloride sheets used for membrane press forming have been finished by hard-coating, such as UV-cure type, which confer stain resistance, scratch resistance and resistance to ultraviolet light on the sheets, which are prepared mainly by a calendering process. However, stripes caused during coating have become a problem when the surface is given a very glossy mirror-like finish. The ultraviolet coating compounds often lack expansion which causes crack to form during membrane press forming and the like. On the other hand, although substrate films such as polyester film may be printed on and transferred to polyvinyl chloride sheets, adhesion of transfer films to the substrate films usually requires heating the back of the substrate film during lamination of the PVC sheets but this leads to a deficit of heat. It is difficult to produce a polyvinyl chloride sheet of sufficient thickness with an even surface by the calendering process alone.